Joint prosthesis

ABSTRACT

A joint prosthesis, especially for a hip joint has a metal shaft receivable in the marrow cavity of the bone, a closure plate at the upper end of the shaft, and a formation adapted to form the ball joint. The underside of the plate and the shaft along its length are provided with a layer of synthetic bone material which is of greater thickness in the region of the plate and adjoining the plate, corresponding to the greater stress region of the bone, than along the shank remote from the plate.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of Ser. No. 07/276,187 filedNov. 23, 1988, now U.S. Pat. No. 4,955,912 issued Sept. 11, 1990.

FIELD OF THE INVENTION

My present invention relates to a joint prosthesis, especially for a hipjoint and, more particularly, to the member of an artificial joint whichis intended to be mounted in the femur of the patient. Specifically, theinvention relates to a member for an artificial hip joint whichcomprises a shaft receivable in the marrow cavity of the femur.

BACKGROUND OF THE INVENTION

In joint prostheses or artificial joints it is common to insert onemember of the artificial joint into a bone forming that joint. In thecase of an artificial hip joint, that member can have a shaft which isforced into the marrow cavity of the femur and is provided at its upperend with a fitting adapted to receive a ball of the joint or forming theball itself. Between the ball and the shaft, a closure plate can beprovided which is adapted to close off the open section of the neck ofthe femur which may be cut away upon removal of the femur head toaccommodate the artificial joint.

The shaft and the plate are generally composed of metal and it has beenproposed to provide at least the shaft with a coating or layer of anartificial bone material, sometimes referred to as synthetic bone or asartificial bone replacement material.

The shaft and the closure plate can be formed together with theball-forming or ball-connecting member in one piece from a titaniumalloy or a cobalt-chromium alloy.

The joint prosthesis can be made by forging, casting and/or machiningoperations.

The layer of the so-called synthetic bone material is generally sinteredon the metal body. The synthetic bone material can have a ceramiccharacter and generally comprises at least one artificial apatite in theform of hydroxyapatite Ca₅ OH(PO₄)₂.

The material can be applied in a multilayer configuration and thevarious layers can be composed of different materials.

The layer of the synthetic bone material is not only highly compatiblewith natural bone tissue, but has a certain porosity.

The shaft which is inserted into the marrow cavity can be provided withribs running in the direction of insertion so as to increase the surfacearea of the layer exposed to the bone tissue. Furthermore, the metallicsurface of the shaft and the closure disc can be treated byspark-discharge erosion to form micropits or roughening formations whichpromote attachment of the synthetic bone material thereto.

A joint prosthesis as thus formed and constituted is not cemented intoor onto the bone as is the case with some artificial joints, but rather,after insertion, becomes permanently attached by a growth of the naturalbone tissue within the cortex of the bone into the layer.

The prosthetic joint of course must have sufficient alternating bendingstrength and should be sufficiently strong as to resist rupture undereven the most extreme conditions of supporting the body.

In practice, the synthetic bone layer has been applied to the metalprosthesis body with a uniform thickness over the entire coated surfacethereof. In practice, moreover, it has been found that the growth ofnatural bone material into the layer and hence the attachment of theprosthesis to the bone is unsatisfactory. Investigations have shown thatthis unexpected phenomenon appears to be a result of the fact thatmovements of the patient cause higher stresses on the bone and theconnection between the bone and the prosthetic member in the region ofthe closure plate than elsewhere along the shaft remote from the closureplate. In other words the more highly stressed region of the prostheticmember and the bone and the more highly loaded tissue at the closureplate and directly adjoining same along the shaft does not permit aseffective growth of the bone tissue into the layer and as effectivebonding of the prosthesis in this region as in regions of reducedloading and less stress more remote from the plate.

The phenomenon, therefore, which has been recognized as a lack ofeffective bonding uniformly between the bone tissue and the implant, hasbeen traced to the sharp differences in loading along the shank or shaftof the implant. This is especially the case with hip prostheses.

OBJECTS OF THE INVENTION

It is, therefore, the principal object of the present invention toprovide an improved prosthesis adapted to be inserted into the marrowcavity of a bone, whereby these drawbacks are avoided.

An object of this invention is to extend the principles set forth in theabove-identified copending application.

Another object of the invention is to provide an improved jointprosthesis, especially for a hip joint, which has improved attachment ofthe implant in the bone tissue, i.e. which will permit bone tissue togrow into the implant more effectively in spite of the fact that thebone and the implant are subjected to greater stress in some regionsthan in others.

Still another object of this invention is to provide an improved hipjoint which is more effectively fixed in the bone.

SUMMARY OF THE INVENTION

These objects and others which will become apparent hereinafter areattained, in accordance with the present invention by providing, in aprosthesis of the type described and especially a hip joint prosthesis,wherein the shaft is provided with a layer of the synthetic bonematerial as has been described, that this layer has a greater thicknessin the higher loading and higher stress regions of the bone than thethickness in the regions of reduced loading. Specifically the greaterthickness should be a multiplicity of times greater than the lesserthickness.

More specifically, a joint prosthesis of the invention can comprise:

an elongated shaft composed of metal and receivable in a marrow cavityof a bone adapted to form one side of a joint;

a closure plate of metal on one end of the shaft and adapted to close anend of the bone, the plate having a surface projecting outwardly at ajunction between the plate and the shaft;

ball-joint means on the closure plate adapted to engage in a socketformation; and

a layer of synthetic bone material covering at least the shaft forenabling bone tissue of the bone into which the shaft is inserted togrow into the layer, the layer having a greater thickness in regions ofthe shaft corresponding to more highly loaded regions of the bone and alesser thickness in regions of the shaft corresponding to lesser loadedregions of the bone, the greater thickness being a multiplicity of timesgreater than the lesser thickness.

Specifically, wherein the joint is a hip prosthesis in which the bone isa femur, the ball-joint means is adapted to form an artificial head ofthe femur, the closure plate is adapted to span across the neck of thefemur, and the layer has the greater thickness adjacent the plate andthe lesser thickness remote from the plate.

According to a feature of the invention, the reduced thickness layer hasa thickness of about 0.2 mm which can correspond to the thickness of thelayer which has been applied uniformly in the past to the implant. Thethickness of the layer in the loaded regions, however, should be 10 to30 times greater than this thickness.

In general it is sufficient to apply the greater thickness of the layerover a region of the shaft extending several centimeters from theclosure plate and, indeed, it has also been found to be advantageous toprovide other regions of high loading with the greater thickness layeras well, e.g. the surface of the plate turned toward the bone tissue,the underside of the plate.

The regions in which higher loading exists can be determined for eachpatient in which the joint prosthesis is to be implanted by techniqueswhich have been developed for determining stresses in bone, namely withX-ray images and the aid of modern computer supported stress mechanicsevaluation. The joint prosthesis can be fabricated to suit eachparticular patient, although it is possible to provide the jointprosthesis in a standard form utilizing the knowledge of the usualstress regions of the bone for the average patient. In either case thereis a marked improvement of the attachment of the prosthesis to the bonetissue by comparison with systems in which the layer of synthetic bonewhich is applied in a uniform thickness over the entire shaft of theprosthesis.

It has been found to be advantageous, moreover, to provide a steplesstransition in thickness between the layer in the more highly loadedregions and the layer in the regions of reduced loading of the bone.

With the joint prosthesis of the present invention there is a highlyintense bonding of the prosthesis in the bone tissue even in the veryhighly stressed regions and the bonding is effected more rapidly inthese regions in spite of the greater stresses to which the prosthesisand the bone tissue may be subject.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features and advantages of my inventionwill become more readily apparent from the following description,reference being made to the accompanying highly diagrammatic drawing inwhich:

FIG. 1 is a diagrammatic view in which the relationship between theparts of the prosthesis have been shown and in which the bone and marrowcavity have been illustrated also in highly diagrammatic form and indot-dash lines;

FIG. 2 is an elevational view, with the layer of synthetic bone materialbroken away, forming a detail of the region III of FIG. 1;

FIG. 3 is a cross-sectional view taken along the line III--III of FIG.1.; and

FIG. 4 is a view similar to FIG. 1 but illustrating an embodiment of theinvention omitting the plate of the embodiment of FIGS. 1-3.

SPECIFIC DESCRIPTION

The hip joint prosthesis illustrated by way of example in FIGS. 1-3 hasa shaft 1 which can be driven into the marrow cavity 10 of a femur 11shown only in dot-lines in the drawing. The shaft 1 may have ribs orother formations 12 and recessed portions 13 to increase the attachmentarea and promote anchoring of the shaft in the bone.

At its upper end, closing a sectioned neck 14 of the femur, the head ofwhich has been surgically removed, is a closure plate 2 from which aball connecting part 3 projects. The ball -6 of the joint has also beenshown in dot-dash lines.

The shaft 1 with the closure plate 2 and the ball connecting part 3 arecomposed of metal, e.g. a titanium or cobalt-chromium alloys has beendescribed.

The shaft 1 and other parts of the prosthesis which may contact bonetissue, are formed with a layer 4 of synthetic bone material which canbe applied after the metal surface has been subjected to spark erosionroughening and sintered in place. The preferred synthetic bone materialforming the layer 4 is hydroxyapatite.

It will be apparent that the shaft 1 inserted into the bone andespecially the underside of the closure plate 2, during movement of thepatient, especially in the case of a hip prosthesis, can be subjected tovastly different stresses and loading. A highly loaded region B at aregion R of reduced loading can thus be distinguished.

From FIGS. 1 and 2 it can be seen that the layer 4 of the synthetic bonematerial, according to the invention varies in thickness along thelength of the shaft and, specifically, as a thickness T in the region ofgreater stress which is some 10 to 30 times greater than the thickness tin the regions of reduced loading. The thickness t can be 0.2 mm. Italso may be seen from the drawing that the region of greater thicknessmay extend from the plate 2 downwardly over a length of severalcentimeters and that there may be a continuous transition in thicknessbetween the two regions.

The downwardly turned surface 18 of the plate 2, i.e. the surface turnedtoward the bone tissue of the neck of the femur, may also be providedwith the layer to the thickness T.

FIG. 4 shows an embodiment similar to that of FIG. 1 but devoid of theplate 2 thereof. As in the earlier embodiment, the hip joint prosthesishas a shaft 1' which is to be driven into the marrow cavity 10 of thefemur 11 and can have the ribs or other formations 12' and recessedportions 13' described in connection with the embodiment of FIG. 1.

In this case, the section neck 14 of the femur is not closed by a plateon the upper end of the shaft 1 but rather has a ball connecting part 3'joined directly to the shaft. The shafts 1 and the ball connecting partare composed of a titanium or cobalt-chromium alloy as described and theshaft 1' is formed with the layer 4' of synthetic bone material of athickness varying along the length of the shaft as described inconnection with FIGS. 1-3.

I claim:
 1. A joint prosthesis, comprising:an elongated shaft composedof metal and receivable in a marrow cavity of a femur adapted to formone side of a joint and having one end proximal to said joint andanother end distal from said joint; ball-joint means on said one end ofsaid shaft adapted to engage in a socket formation; and a layer ofsynthetic bone material covering at least said shaft for enabling bonetissue of the femur into which said shaft is inserted to grow into saidlayer, said layer having a greater thickness in regions of said shaftproximal to said one end and a lesser thickness in a region of saidshaft, said greater thickness being a multiplicity of times greater thansaid lesser thickness, the synthetic bone material is hydroxyapatite andis sintered to said shaft, said shaft being composed of a titanium orcobalt-chromium alloy, said lesser thickness being about 0.2 mm and saidgreater thickness being about 10 to 30 times greater than said lesserthickness.
 2. The joint prosthesis defined in claim 1 wherein said layerhas said greater thickness over several centimeters of the length ofsaid shaft extending from said one end.
 3. The joint prosthesis definedin claim 1 wherein the greater thickness of said layer merges steplesslywith the lesser thickness of said layer.